(1) Field of the Invention
The present invention relates to secret speech equipment for ensuring the secrecy of analog voice signals, and more particularly, to secret speech equipment for carrying out a band split frequency scrambling after a digital signal processing of the analog voice signal.
Namely, the present invention relates to a speech scrambler or communication security equipment in which input sampling signals are converted into low-speed sampling signals, and the low-speed sampling signals are then subjected to a digital signal processing and frequency split and permutation.
The analog scrambling technology has long been utilized to ensure speech privacy, and this technology is now widely used in voice communication systems utilizing analog channels such as analog telephones and mobile radio systems, but in these voice communication systems, the bandwidth of the scrambled voice should not be allowed to expand, and thus most scrambling technologies provide an unsatisfactory level of security for the scrambled voice: Even if a high level of security can be guaranteed, the quality of the unscrambled voice is not always good and the cost is high.
(2) Description of the Related Art
One type of known conventional analog speech scrambler is a frequency split and permutation equipment. In such a conventional analog speech scrambler, the input speech band is split by analog band-pass filters, and the respective split bands are permutated by converting the frequencies by modulators and by carrying out an inverse conversion by demodulators, and thus the circuit scale is unavoidably enlarged.
Accordingly, in a currently used speech scrambler, an A/D conversion of the input analog signals is carried out, and then a frequency split and permutation are carried out by a digital filter bank.
In a conventional speech scrambler employing such a digital filter bank, since the signal processing rate for each frequency band is equal to the sampling rate of the input signal, a disadvantage occurs in that the number of split bands must be increased and, therefore, the amount of processed signals becomes large when the security level of a cryptogram is raised.
That is, when the number of split or divided bands is increased, to ensure a greater speech secrecy, the number of digital filters must be increased accordingly, and since filters having sharp cutoff characteristics are required, the number of filter taps is increased when the band width is narrowed. As a result, a problem arises in that the total amount of signal processing is increased.